Google Trends for 'iPhone slow' clearly shows a periodic spike in interest.
These spikes occur in September 2013, September 2014, September 2015, ....
The release schedule for iPhones is September 2013, September 2014, September 2015, ....
Is it coincidence that people search for 'iPhone slow' around the time of a new iPhone release? It seems not. Apple have now admitted (or partially admitted) that they do slow down older models, according to a BBC report they claim it was "to prolong the life of the devices". Other people suspect Apple deliberately slowed older models just before the release of each new iPhone to encourage iPhone users to upgrade.
In the Google Trends graph above interest searches for 'iPhone slow' rapidly spiked they gradually fell away, but in 2017 the pattern changed, the searches didn't fall away they continued and grew stronger. More people were perhaps becoming aware of what Apple has been up to.